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Optimize ASCII digit and hexdigit conversion by using subtraction #21

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Optimize ASCII digit and hexdigit conversion by using subtraction #21

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70a81bc
Optimize ASCII digit & hexdigit using subtraction
okaneco Dec 8, 2022
e25676c
Optimize hex conversion further, Add more tests
okaneco Jan 14, 2023
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101 changes: 62 additions & 39 deletions src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -23,7 +23,7 @@

use num_traits::{
ops::checked::{CheckedAdd, CheckedMul},
Bounded, CheckedSub, One, Signed, Zero,
Bounded, CheckedSub, FromPrimitive, One, Signed, Zero,
};
use core::{
cmp::{max, min},
Expand Down Expand Up @@ -332,26 +332,18 @@ where
/// ```
pub fn ascii_to_digit<I>(character: u8) -> Option<I>
where
I: Zero + One,
I: Zero + One + FromPrimitive,
{
match character {
b'0' => Some(nth(0)),
b'1' => Some(nth(1)),
b'2' => Some(nth(2)),
b'3' => Some(nth(3)),
b'4' => Some(nth(4)),
b'5' => Some(nth(5)),
b'6' => Some(nth(6)),
b'7' => Some(nth(7)),
b'8' => Some(nth(8)),
b'9' => Some(nth(9)),
_ => None,
if (b'0'..=b'9').contains(&character) {
I::from_u8(character - b'0')
} else {
None
}
}

impl<I> FromRadix10 for I
where
I: Zero + One + AddAssign + MulAssign,
I: Zero + One + FromPrimitive + AddAssign + MulAssign,
{
fn from_radix_10(text: &[u8]) -> (Self, usize) {
let mut index = 0;
Expand All @@ -371,7 +363,7 @@ where

impl<I> FromRadix10Signed for I
where
I: Zero + One + AddAssign + SubAssign + MulAssign,
I: Zero + One + AddAssign + SubAssign + MulAssign + FromPrimitive,
{
fn from_radix_10_signed(text: &[u8]) -> (Self, usize) {
let mut index;
Expand Down Expand Up @@ -428,7 +420,8 @@ where
+ CheckedAdd
+ CheckedSub
+ CheckedMul
+ MaxNumDigits,
+ MaxNumDigits
+ FromPrimitive,
{
fn from_radix_10_signed_checked(text: &[u8]) -> (Option<Self>, usize) {
let mut index;
Expand Down Expand Up @@ -503,7 +496,7 @@ where

impl<I> FromRadix10Checked for I
where
I: Zero + One + FromRadix10 + CheckedMul + CheckedAdd + MaxNumDigits,
I: Zero + One + FromRadix10 + CheckedMul + CheckedAdd + MaxNumDigits + FromPrimitive,
{
fn from_radix_10_checked(text: &[u8]) -> (Option<I>, usize) {
let max_safe_digits = max(1, I::max_num_digits_negative(nth(10))) - 1;
Expand All @@ -526,32 +519,29 @@ where
/// Converts an ascii character to digit
fn ascii_to_hexdigit<I>(character: u8) -> Option<I>
where
I: Zero + One,
I: Zero + One + FromPrimitive,
{
match character {
b'0' => Some(nth(0)),
b'1' => Some(nth(1)),
b'2' => Some(nth(2)),
b'3' => Some(nth(3)),
b'4' => Some(nth(4)),
b'5' => Some(nth(5)),
b'6' => Some(nth(6)),
b'7' => Some(nth(7)),
b'8' => Some(nth(8)),
b'9' => Some(nth(9)),
b'a' | b'A' => Some(nth(10)),
b'b' | b'B' => Some(nth(11)),
b'c' | b'C' => Some(nth(12)),
b'd' | b'D' => Some(nth(13)),
b'e' | b'E' => Some(nth(14)),
b'f' | b'F' => Some(nth(15)),
_ => None,
// Unsetting the 6th bit converts ASCII alphabetic lowercase to uppercase.
//
// b'A' = 0b_0100_0001 (decimal 65), b'F' = 0b_0100_0110 (decimal 70)
// b'a' = 0b_0110_0001 (decimal 97), b'f' = 0b_0110_0110 (decimal 102)
// b'a' & 0b_1101_1111 converts 'a' to 'A'.
let mask = 0b_1101_1111;

if matches!(character, b'0'..=b'9') {
I::from_u8(character - b'0')
} else if matches!(character & mask, b'A'..=b'F') {
// Subtract 55 from the result to map the character to its hexadecimal
// value: (65 to 70) - 55 => 10 to 15
I::from_u8((character & mask) - 55)
} else {
None
}
}

impl<I> FromRadix16 for I
where
I: Zero + One + AddAssign + MulAssign,
I: Zero + One + FromPrimitive + AddAssign + MulAssign,
{
fn from_radix_16(text: &[u8]) -> (Self, usize) {
let mut index = 0;
Expand All @@ -571,7 +561,7 @@ where

impl<I> FromRadix16Checked for I
where
I: Zero + One + FromRadix16 + CheckedMul + CheckedAdd + MaxNumDigits,
I: Zero + One + FromRadix16 + CheckedMul + CheckedAdd + MaxNumDigits + FromPrimitive,
{
fn from_radix_16_checked(text: &[u8]) -> (Option<I>, usize) {
let max_safe_digits = max(1, I::max_num_digits_negative(nth(10))) - 1;
Expand Down Expand Up @@ -684,4 +674,37 @@ mod test {
assert_eq!((Some(25), 2), u8::from_radix_16_checked(b"19"));
assert_eq!((Some(25), 2), u8::from_radix_16_checked(b"19!Blub"));
}

#[test]
fn ascii_to_digit_check() {
for (decimal, byte) in (0..=9).zip(b'0'..=b'9') {
assert_eq!(Some(decimal), ascii_to_digit(byte));
}

// Assert that 0..=255 only returns valid decimals
let decimals = 0_u8..=9;
assert!((u8::MIN..=u8::MAX)
.filter_map(|n| ascii_to_digit::<u8>(n))
.eq(decimals));
}

#[test]
fn ascii_to_hexdigit_check() {
for (decimal, byte) in (0x0..=0x9).zip(b'0'..=b'9') {
assert_eq!(Some(decimal), ascii_to_hexdigit(byte));
}
for (decimal, byte) in (0xA..=0xF).zip(b'A'..=b'F') {
assert_eq!(Some(decimal), ascii_to_hexdigit(byte));
}
for (decimal, byte) in (0xA..=0xF).zip(b'a'..=b'f') {
assert_eq!(Some(decimal), ascii_to_hexdigit(byte));
}

// Iterator over the hexadecimals '0x0'..='0x9':'0xA'..='0xF',
// chaining '0xA'..='0xF' an additional time for lowercase b'a'..=b'f'
let hexadecimals = (0_u8..=9).chain(0xA..=0xF).chain(0xA..=0xF);
assert!((u8::MIN..=u8::MAX)
.filter_map(|n| ascii_to_hexdigit::<u8>(n))
.eq(hexadecimals));
}
}